The molecular weight effects of poly(acrylic acid) on calcium carbonate inhibition in the kraft pulping process.

Abstract

Calcium carbonate scale reduces process efficiency and control in kraft pulping operations. The formation of scale can be alleviated or at least reduced by the addition of antiscalant chemicals into the process line. A number of different antiscalants are commercially available but still little is known about their mechanism of inhibiting scale formation. This project focused on one class of antiscalants, the polymeric antiscalants, which are carboxylic acid containing copolymers, with the majority of their monomer residues being poly(acrylic acid). Scale inhibition performance at high pH and temperatures was characterized using a number of different experimental setups and the mechanisms involved were investigated. Performance tests yielded that the most influential characteristic of the polymeric antiscalants was the molecular weight (MW), and consequently, the molecular weight distribution (MWD) of the polymer species. Since commercial poly(acrylic acid) (PAA) samples have a broad MWD, atom transfer radical polymerization (ATRP) was utilized to create relatively monodisperse samples of PAA. These PAA samples synthesized via ATRP were used to investigate the effects of MW and MWD on antiscalant performance along with the mechanisms responsible for the observed optimal MW range of ~10,000 under laboratory kraft pulping conditions. The information presented here is useful both in identifying important properties of polymeric antiscalants and in understanding the mechanisms by which they inhibit the formation of calcium carbonate crystalline deposits in kraft pulping.